Fluent Validation in Asp.Net API

FluentValidation is a popular validation library for .NET applications. It provides a fluent and expressive syntax for defining validation rules for our models. With FluentValidation, we can easily validate user input and ensure data integrity, reducing the chance of errors and improving the overall quality of our application. It offers a wide range of validaton rules and features, including-

• Customizable Validation Rules
• Property-Level and Cross-Property Validation
• Error Messages and Localization
• Rule sets - group validation rules together
• Asynchronous Validation
• Custom Error Codes and Severity Level
• Testing Support and more It integrates seamlessly with ASP.NET core, making it a versatile choice for implementing robust and maintainable validation logic in our .NET projects.

1. Create a folder for the project and get into the directory

   mkdir FluentValidationAPI
   cd FluentValidationAPI
   

2. Create a solution named FluentValidationAPI

   dotnet new  sln -n FluentValidationAPI
   

3. Create a project into the folder

   dotnet new webapi -f net6.0 -n FluentValidationAPI
   

4. Add the project into the solution

   dotnet sln add FluentValidationAPI/FluentValidationAPI.csproj
   

5. Open the solution with visual studio code

   code .
   

6. Add a gitignore file into the solution

   dotnet new gitignore
   

7. Add a README.md file into the solution

   touch README.md
   

8. Install the following packages

   dotnet add package FluentValidation -v 11.9.0  
   dotnet add package FluentValidation.AspNetCore -v 11.3.0  
   dotnet add package FluentValidation.DependencyInjectionExtensions -v 11.9.0
   

9. Inject FluentValidationAutoValidation into services in program file

   builder.Services.AddFluentValidationAutoValidation();
   

10. Create a folder named Models and a model class named User

    mkdir Models
    cd Models
    dotnet new class -n User
    

    public class User
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }
    

11. Now create a validator class named UserValidator

    dotnet new class -n UserValidator
    

    public class UserValidator : AbstractValidator<User>
    {
        public UserValidator()
        {
            RuleFor(user => user.Name).NotEmpty().WithMessage("Name is required.");
            RuleFor(user => user.Age).InclusiveBetween(18, 99).WithMessage("Age must be between 18 and 99.");
        }
    }
    

12. Register the validator into program file

    builder.Services.AddScoped<IValidator<User>, UserValidator>();
    

13. Now create a new POST method in the WeatherForecastController

    // Automatic validation
    [HttpPost]
    public IActionResult Create(User model)
    {
        if (!ModelState.IsValid)
        {
            return StatusCode(StatusCodes.Status400BadRequest, ModelState);
        }
    
        // Continue with user creation process
    
        return StatusCode(StatusCodes.Status201Created, "User created successfully!");
    }
    

14. We have the option to specifically designate the class or model for which we want to create the validator. In this scenario, Validate method would provide ValidationResult object with two key attributes. The first is IsValid, a boolean indicating the success or failure of the validation process. The second attribute is Errors, which comprises a list of ValidationFailure objects containing detailed information about any encountered errors. For that we will do the implementation such following-

    // Manual validation
    [HttpPut]
    public IActionResult Update(User user)
    {
        var validationResult = _userValidator.Validate(user);
    
        if (!validationResult.IsValid)
        {
            return StatusCode(StatusCodes.Status400BadRequest, validationResult.Errors);
        }
    
        // Continue with user update process
    
        return StatusCode(StatusCodes.Status200OK, "User updated successfully!");
    }
    

    Now test the new PUT request and check if validators work or not.

Reference : https://dev.to/andytechdev/how-to-use-fluentvalidation-in-aspnet-core-net-6-2fnf